The invention applies generally to the stripping of an electric-conducting element or substrate of any shape, covered with a somewhat flat protective and/or insulating element, in order to reach said electric-conducting element or substrate so as it can be connected to other circuits, and applies more specifically to stripping shielded and sheathed cables in the description disclosed herein.
Such cables are made of several conducting wires insulated by means of a lateral sheath. The conducting wires, placed side by side so as to form a bundle, are shielded and covered with a protective and insulating sheath.
In order to use and connect the cables, it is necessary to strip their end portions. This stripping operation involves removing the outer protective sheath on a portion of the cable length so as to expose the end of the shield.
For quality, reliability and safety reasons, it is essential that the shield should not be damaged even superficially during the stripping operation.
The present invention is designed to improve the stripping quality of such cables and more precisely to prevent any contact or deterioration of the shield.
At present, shielded or non-shielded cables are manually stripped (by means of a scalpel, mechanical or thermal nippers) or automatically stripped using mechanical or thermal means.
The implementation of such known techniques does not fully ensure that the wire conductor and/or the shield of the cables has suffered no partial break, contact, cut nor any other deterioration caused by such process, since the process does not include any self-check.
As far as manual stripping is concerned, involving the use of a scalpel for example, cutting depends on the operator's uncheckable skill.
As for mechanical and automatic stripping, for example using rotary knives or shaping knives, cutting depends on the constant adjustment of the depth that has been initially determined by the operator, an adjustment that does not change during the stripping operation and thereby does not take possible variations into account.
As for thermal stripping, metal heating clamping jaws that scorch the insulating cover by successive impressions are used. The result depends on the operator's uncheckable skill like for the scalpel, since the clamping jaws can contact and deteriorate the wire conductor or the shield thereof.
Moreover, temperature cannot be controlled, neither the pressure nor the duration, depending on the variation in thickness and in shape of the insulating means or sheaths to strip. Furthermore, such process emits noxious vapors liable to pollute the atmosphere. Finally, the cutting of the insulating materials is not clear nor sharp because of scorching.
Among automatic methods, the cutting method by means of a laser beam focussed on the area of the insulant to be scorched should be mentioned. Yet, the need to focus the laser beam very precisely onto the area to be cut precludes the use of such methods for stripping outer sheaths since they are not rotationally symetrical as shielded and sheathed cables are. Besides, such method does not guarantee that the parts that are subjacent to the outer sheaths are free from any contact or deterioration.
It is an object of the invention to remedy the disadvantages of the known aforesaid stripping techniques by providing a device that is able to guarantee the stripping of such cables, thanks to a continuous self-check, while preserving the integrity of the exposed shield.
Self-checked cables stripping processes are already known. For example, the U.S. Pat. No. 4,999,910 describes the improvements of a stripping apparatus comprising two diametrically-opposed blades mounted as a clamp and moving simultaneously toward each other, the improvements brought by this patent consisting, briefly, in applying a voltage to the blades and detecting a current transfer between the blades and the core wire of the cable when the blades come near said core wire, such detection bringing the blades to a halt.
However, such method shows serious disadvantages.
It is necessary to apply a high voltage to the blades to obtain a noticeable load. In the example described in the aforesaid U.S. patent, a 300 V voltage is applied to the blades. The process requires taking precautions, and more precisely, to insulate the blades and the driving system so as to prevent the operator from receiving discharges.
Besides, the quality of the electric discharge between the blades and the core wire and more precisely the intensity, directly depends on the initial electric state of the core wire, so that one is never really sure that the blades are stopped soon enough, that is to say that they have not started to cut the outer surface of the core wire.
Finally, the current loads stored on the core wire after stripping should be removed so that the core wire returns to a voltage-neutral condition, which is imperative as emphasized in the description (col. 12, lines 49-57) in order to avoid problems in the electric circuits wherein the stripped cables shall be used and so that the other end of the same cable can be stripped similarly.
It also should be noted that the apparatus described in the U.S. Pat. No. 4,999,910 is inoperative for stripping non-circular outer profile cables because of the cutting methods having two diametrically-opposed blades travelling simultaneously and synchronously.